Low gate current field emitter cell and array with vertical thin-film-edge emitter

ABSTRACT

A field emitter cell includes a thin-film-edge emitter normal to the gate layer. The field emitter cell may include a conductive substrate layer, an insulator layer having a perforation, a gate layer having a perforation, an emitter layer, and other optional layers. The perforation in the gate layer is larger and concentrically offset with respect to the perforation in the insulating layer and may be of a tapered construction. Alternatively, the perforation in the gate layer may be coincident with, or larger or smaller than, the perforation in the insulating layer, provided that the gate layer is shielded from the emitter from a direct line-of-sight by a nonconducting standoff layer. Optionally, the thin-film-edge emitter may include incorporated nanofilaments. The field emitter cell has low gate current, making it useful for various applications such as field emitter displays, high voltage power switching, microwave, RF amplification and other applications that require high emission currents.

[0001] This is a continuation-in-part application of copending U.S.patent application Ser. No. 09/478,899, inventors Hsu et al., filed Jan.7, 2000.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates generally to field emitter cellsand arrays. More specifically, the present invention relates to low gatecurrent thin-film-edge emitter cells and arrays.

[0004] 2. Description of the Background Art

[0005] Very small localized vacuum electron sources which emitsufficiently high currents for practical applications are difficult tofabricate. This is particularly true when the sources are required tooperate at reasonably low voltages. Presently available thermionicsources do not emit high current densities, but rather result in smallcurrents being generated from small areas. In addition, thermionicsources must be heated, requiring special heating circuits and powersupplies. Photo emitters have similar problems with regard to lowcurrents and current densities.

[0006] Field emitter arrays (FEAs) are naturally small structures whichprovide reasonably high current densities at low voltages. FEAstypically comprise an array of conical, pyramidal or cusp-shaped point,edge or wedge-shaped vertical structures which are electricallyinsulated from a positively charged extraction gate and which produce anelectron beam that travels through an associated opening in the chargedgate.

[0007] The classical field emitter includes a sharp point at the tip ofthe vertical structure and opposite an extraction electrode. In order togenerate electrons which are not collected at the extraction electrode,but can be manipulated and collected somewhere else, an aperture iscreated in the extraction electrode which aperture is significantlylarger (e.g. two orders of magnitude) than the radius of curvature ofthe field emitter. Thus, the extraction electrode is a flat horizontalsurface containing an extraction electrode aperture for the fieldemitter. The field emitter is centered horizontally in the extractionelectrode aperture and does not touch the extraction electrode, althoughthe vertical direction of the field emitter is perpendicular to thehorizontal plane of the extraction electrode. The positive charges onthe edge of the extraction electrode aperture surround the field emittersymmetrically so that the electric field produced between the fieldemitter and the extraction electrode causes the electrons to be emittedfrom the field emitter in a direction such that are collected on anelectrode (anode) separate and distinct from the extraction electrode. Avery small percentage of the electrons are intercepted by the extractionelectrode. The smaller the aperture, i.e., the closer the extractionelectrode is to the field emitter, the lower the voltage required toproduce field emission of electrons.

[0008] It is difficult to create FEAs which have reproducibly smallradius-of-curvature field emitter tips of conducting materials orsemiconducting materials. Furthermore, it is equally difficult to gateor grid these structures where the gate-to-emitter distance isreasonably small to provide the necessary high electrostatic field atthe field emitter tip with reasonably small voltages. The radius ofcurvature is typically 100-300 angstroms (Å) and the gate-to-emitterdistance is typically 0.1-0.5 micrometers (μm).

[0009] Current methods of manufacturing FEAs include wet etching,reactive ion etching, and a variety of field emitter tip depositiontechniques. Practical methods generally require the use of lithographywhich has a number of inherent disadvantages including the high cost ofthe equipment needed. Furthermore, the high degree of spatialregistration requires expensive high resolution lithography.

[0010] To a large extent, these prior art problems were overcome by Hsuet al., U.S. Pat. No. 5,584,740 and Gray et al., U.S. Pat. No.5,382,185, both of which are incorporated herein by reference for allpurposes in their entirety. The '740 and '185 patents describe athin-film-edge emitter cell including a substrate having a protuberanceextending therefrom, a conformally deposited insulating layer over thesubstrate and vertical sidewall of the protuberance, an emitter filmconformally deposited upon the insulating layer and the verticalsidewall thereof, and a gate metallization layer parallel to thevertically extending portion of the emitter film. The emitter filmextends vertically beyond the protuberance. U.S. Pat. Nos. 5,214,347 and5,266,155 to Gray, both are which are incorporated-by-reference hereinin their entirety for all purposes, describe horizontal thin-film edgefield emitters and gated field emitters.

[0011] Because of the parallel orientation of the emitter film relativeto the gate, the insulating layer between these elements in thosepatented devices must be sufficiently thin so that, at the emitter tip,the gate generates a field capable of extracting electrons at the tip.The dependence of the gate to tip distance upon insulating filmthickness requires a trade off between the reduced susceptibility topinhole defects and voltage breakdown offered by thicker insulatingfilms and the increased voltage demands caused by the resultingadditional gate to tip distance. Additionally, the parallel orientationof the gate layer creates a high capacitance. In turn, this highcapacitance increases the RC time constant, reducing frequency responseand power efficiency.

[0012] Commonly-owned U.S. application Ser. No. 09/045,853 filed on Mar.23, 1998, which is incorporated herein by reference in its entirety,describes an improved field emitter cell/array that can potentiallysurpass the current conical or pyramidal tip FEAs both in terms ofperformance and cost. In particular, the FEAs described in the '853application provides various performance advantages including higheremission current, lower voltage, lower capacitance, highertransconductance, resistance to “poisoning” by ambient gas, resistanceto oxidation and resistance to blunting by back-ion bombardment. Infact, we have found that the FEAs of the '853 application has betteroxidation resistance than any FEA known to date. Furthermore, the FEAsof the '853 application can be manufactured at relatively low cost ascompared to prior devices. For example, the manufacture of FEAs of the'853 application requires only about one-third as many processing stepsas the conical tip FEAs. In addition, lithography can be replaced withstamping technology in making the one-step masks for producing thestarting template structures described in the '853 application.

[0013] However, the FEAs disclosed in the '853 application typicallyexhibit gate currents of about 7 to about 15% of the anode current.Although gate currents of these sizes are acceptable for applicationsthat involve low currents and low power levels (e.g., field emitterdisplays), they cannot be tolerated in applications that require highcurrents and high power levels (e.g., power switching, microwave, andmillimeter wave power amplifiers). Specifically, power dissipation atgreater than about 1% gate current would damage the FEA.

SUMMARY OF THE INVENTION

[0014] Accordingly, it is an object of the present invention to providea field emitter cell that exhibits significantly reduced gate currents.

[0015] It is another object of the present invention to drasticallyreduce the gate current in field emitter cells to as low as less thanabout 1% of the anode current.

[0016] It is a further object of the present invention to provide afield emitter cell that retains all the advantages (e.g., relatively lowmanufacturing cost, high emission current, low voltage, low capacitance,high transconductance, resistance to “poisoning” by ambient gas,resistance to oxidation and resistance to blunting by back-ionbombardment) of the field emitter cells described in U.S. applicationSer. No. 09/045,853, and yet exhibit very low gate currents.

[0017] It is yet another object of the present invention to providefield emitter cells that are suitable for various applications includinghigh voltage power switching, microwave, RF amplification and otherapplications that require high emission currents.

[0018] These and other objects of the invention are accomplished by afield emitter cell comprising:

[0019] an electrically conductive substrate layer;

[0020] an insulating layer directly upon said electrically conductivesubstrate layer; said insulating layer having a first perforationtherethrough, said first perforation having an aperture, at least oneessentially vertical sidewall and a bottom surface defined by saidelectrically conductive substrate layer;

[0021] an electrically conductive gate layer directly upon saidinsulating layer, said electrically conductive gate layer having asecond perforation therein, said second perforation having an aperturelarger than the aperture of said underlying first perforation, and

[0022] an electrically conductive thin film edge emitter, electricallyinsulated from said gate layer and in electrical contact with saidsubstrate layer, said emitter extending upward from at least within saidfirst perforation and essentially parallel to said side walls, saidemitter having an upper electron-emitting edge in close proximity tosaid gate layer, said electrically conductive thin film edge emitterforming a shell having said upper electron-emitting edge as an openupper end of said shell. If desired, the second perforation may betapered; that is, the aperture of the second perforation may have atapered shape. However, the opening of the second perforation may benon-tapered, where its sidewalls are parallel to the vertical sidewallsof the first perforation. If the perforations are cylindrical, thesecond perforation may be concentric with the first perforation.

[0023] In another embodiment, the field emitter cell of the presentinvention comprises:

[0024] an electrically conductive substrate layer,

[0025] an insulating layer directly upon said electrically conductivesubstrate layer, said insulating layer having a first perforationtherethrough, said first perforation having at least one essentiallyvertical sidewall and a bottom surface defined by said electricallyconductive substrate layer;

[0026] an electrically conductive gate layer directly upon saidinsulating layer, said electrically conductive layer having a secondperforation therein, said second perforation being coincident with, orlarger or smaller than, said underlying first perforation;

[0027] an electrically conductive thin film edge emitter, electricallyinsulated from said gate layer and in electrical contact with saidsubstrate layer, said emitter extending upward from at least within saidfirst perforation and essentially parallel to said side walls, saidemitter having an upper electron-emitting edge in close proximity tosaid gate layer, said electrically conductive thin film edge emitterforming a shell having said upper electron-emitting edge as an openupper end of said shell; and

[0028] a standoff (or spacer) layer extending upward from within saidfirst perforation and essentially parallel to said side walls, saidstandoff layer being disposed between said emitter and said verticalside walls and having an upper portion that substantially shields saidgate layer from said emitter.

[0029] In all embodiments of the present invention, the emitter layer(s)may optionally incorporate nanofilaments, which are usually hollow orsolid, high-aspect-ratio needle-like structures. Furthermore, in allembodiments, the emitter layer(s) may extend from below the upperhorizontal surface of the conducting substrate in cases where standofflayer(s) is/are used and the substrate is provided with a cavity withlateral dimensions greater than that of the inner dimension of theaperture defined by the bottom portion of the standoff layer(s).

[0030] The field emitter cell of the present invention may be made byvarious methods using known lithographic, deposition, and etching steps.

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] A more complete appreciation of the invention will be readilyobtained by reference to the following Description of the PreferredEmbodiments and the accompanying drawings in which like numerals indifferent figures represent the same structures or elements, wherein:

[0032]FIGS. 1 and 2 show the side (cross-sectional) and top views,respectively, of a field emitter cell according to the present inventionin which the gate layer aperture is tapered.

[0033]FIGS. 1A shows the side (cross-sectional) view of a field emittercells according to the present invention in which the gate layeraperture is tapered and the conducting substrate is provided with acavity with lateral dimensions greater than that of the inner dimensionof the aperture defined by the bottom portion of the standoff layer(s).

[0034]FIGS. 3 through 7 show the processing steps involved infabricating a field emitter cell according to the present invention inwhich the gate layer aperture is tapered.

[0035]FIGS. 5A through 7A show the processing steps involved infabricating field emitter cells according to the present invention inwhich the conducting substrate is provided with a cavity with lateraldimensions greater than that of the inner dimension of the aperturedefined by the bottom portion of the standoff layer(s).

[0036]FIGS. 8 and 9 generally show the side (cross-sectional) and topviews, respectively, of a field emitter cell according to the presentinvention in which the gate layer aperture is offset with respect to theinsulating layer aperture.

[0037]FIGS. 10 through 14 show the processing steps involved infabricating a field emitter cell according to the present invention inwhich the gate layer aperture is non-tapered and offset with respect tothe insulating layer aperture and in which the gate layer is notetchable by standard dry etching.

[0038]FIGS. 15 through 19 show the processing steps involved infabricating a field emitter cell according to the present invention inwhich the gate layer aperture is non-tapered and offset with respect tothe insulating layer aperture and in which the gate layer is etchable bystandard dry etching.

[0039]FIGS. 20 through 24 show the processing steps involved infabricating a field emitter cell according to the present invention inwhich the gate layer aperture is coincident with the insulating layeraperture and in which a standoff layer is used to shield the gate layerfrom the emitter.

[0040]FIG. 25 is an electron micrograph of a field emitter cell having atapered gate layer aperture.

[0041]FIG. 26 is a graph showing the emission current versus gatevoltage for a field emitter cell according to the present invention.

[0042]FIG. 27 is a graph showing the emission current versus gatevoltage for yet another emitter cell according to the present invention.

[0043]FIGS. 28 and 29 show the side (cross-sectional) and top views of acell wherein the emitter layer incorporates nanofilaments without acapping (or sandwiching) conducting protection layer for thenanofilaments.

[0044]FIGS. 30 and 31 show the side (cross-sectional) and top views of acell wherein the emitter layer incorporates nanofilaments with a capping(or sandwiching) conducting protection layer for the nanofilaments.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0045] The present invention includes a field emitter cell in which thethin-film-edge emitter is essentially perpendicular to the gate layer,insulation layer, and substrate. That unique arrangement maximizes thedistance between the gate and substrate, and minimizes the distancebetween the gate and the emitter. The arbitrarily tall height of theemitter minimizes the capacitance, resulting in a low RC constant whichenhances power efficiency in high frequency applications. Unlike priorart field emitter cells, the dimensions of the components of the fieldemitter cell are independent of each other. That is, in previously knownFEAs such as conical FEAs, the component dimensions are mutuallylimiting.

[0046] Importantly, the significant reduction in the gate currentachieved by the present invention is generally obtained by arranging thegate layer and the insulator layer(s) so that the insulator and/orstandoff layer(s) block the gate layer from any direct line-of-sightfrom the emitter. Typically, the insulator layer near the region of thegate layer aperture becomes negatively charged. For example, thereduction in the gate current may be achieved by using a gate layer inwhich the perforation therein has an aperture that is larger in sizethan the aperture of the perforation in the insulating layer. In thisway, a portion of the insulating layer can serve to substantially shieldthe gate layer from the emitter. Alternatively, a standoff (or spacer)layer may be used to insulate or shield essentially the emitter from adirect line-of-sight from the gate layer, While not wishing to be boundby any theory, it is believed that the reduced gate current arises fromthe emitted electrons having to travel well above the substrate (as aresult of the shielding by the insulator and the negative charge on theinsulator layer in a region near the gate layer) before there exists adirect line-of-sight from the electrons to the gate, at which point theelectrons would be under the influence of the potential created by theanode electrode rather than the gate layer. In general, heat dissipationthrough the gate and associated degradation are lowered as the gatecurrent is reduced.

[0047] The substrate layer can be a conductor, an n-doped semiconductor,a combination of n-doped and p-doped semiconductors, a resistivematerial, a transistor, or a composite, alloy, or multilayer structureincluding one or more of these classes of materials. The substratelayer, however, should be capable of conducting charge, with or withoutconduction control elements by which charge conduction can be controlledby individual or group-addressable means such as electrical, magnetic,optical, or mechanical methods. Throughout the present invention andclaims, the terms “conductor” and “conducting material” include bothnormal conductors and superconductors unless otherwise stated. If aresistive material is used as the substrate layer, emission current canbe controlled or limited to prevent burnout of the emitter and toprovide emission area uniformity as well as a decrease in noise. Aresistive material minimizes burnout by causing an IR drop in potentialthat results in current limiting in the field emitter cell.

[0048] The insulating layer that overlies the substrate, and underliesthe gate layer, may be any electrically insulating material. Typicalmaterials useful as the insulating layer of the present inventioninclude, but are not limited to, oxides, diamond, glass, and organicmaterials (e.g., organic polymers).

[0049] In one preferred embodiment, the arrangement of the gate layerand the insulator layers is such that the insulating layer is capable ofcapturing electrons emitted from the emitter and become negativelycharged, thereby shielding the emitted electrons from being subject tothe positive gate charge in the local region close to the gate and thusrepelling them into trajectories away from the gate and more towards theanode. Another benefit of this local negative charge is that the emitterwill be protected from bombardment by residual ions. Positive ions(resulting from electron ionization of ambient residual gases) that arenear these local negatively charged regions will be attracted towardsthese regions and land on them rather than impinging on the emitters.

[0050] The sidewalls of the first perforation in the insulating layer inthe present invention should be essentially vertical. Typically, thesewalls extend at an angle of at least about 80° (and more often at anangle of at least 85°) with respect to the substrate and preferablyextend at an angle of substantially 90° with respect to the substrate.Because of its superior electrical and mechanical properties, acylindrical structure is most typical, but is not required for thepractice of this invention. Any other shape (e.g, a shape having asquare, rectangular, zig-zig, spiral, etc. cross-section) may be used.

[0051] The emitter is preferably any low work function material that isprotected from ready oxidation. Typically, the emitter is selected fromamong the same class of materials as is the substrate layer. The emitterlayer should be capable of conducting electrons. A preferred conductingmaterial is lithium sandwiched between platinum layers, although othermaterials can be readily used. Typically, materials useful as theemitter include platinum, its compounds and its alloys; osmium, itscompounds and its alloys; ruthenium, its compounds and its alloys; andlithium, its compounds and its alloys. Other suitable materials includeW, Mo, Ni, Ti, Cr and carbon (with or without a diamond-like structure)having significant sp² content. Also, compound materials such as siliconcarbide and transition metal carbides such as zirconium carbide, hafniumcarbide, titanium carbide, and transition metal nitrides may be used.The emitter, like the substrate, may also be an inhomogeneous compositeor a multilayer structure. Typically, when the emitter is an alloy,composite (mixture or inhomogeneous) or multilayer structure, at leastone of the materials typically has a low work function. For example,alloys of Li with Pt and/or Ru are useful as an emitter material in thepresent invention. Typically, a path for electron conduction should beprovided between the substrate and the emitter. If a multilayerstructure is used, only one of the layers need to be electronconductive; the one layer can be a conductor, a semiconductor, aresistive layer, or a transistor. This layer is in electrical contactwith the substrate. The other layers can be any other suitable material,e.g., a nonconducting layer. If a resistive material is used for theemitter, the emission current can be controlled to prevent emitterburnout and to provide area uniformity as well as a decrease in noise. Aresistive material minimizes burnout by causing an IR drop in potentialthat results in current limiting in the field emitter cell. To be usefulas a current-limiting resistive material, the resistivity must be highenough that the resulting IR drop is a significant fraction of the totalpotential between the emitter and the gate.

[0052] In one particularly preferred embodiment the emitter may be anoble metal/low work function material/noble metal sandwich, typicallywith each layer of the sandwich having a thickness of about 0.005 toabout 0.1 microns. For example, Ru/Li/Ru and Pt/Li/Pt sandwiches haveprovided excellent results. Other noble metals useful as outer layers(layers covering the emitter layer except the top emitting edge) in anemitter structure for the present invention include Pd, Au, Ir and Os.Non-noble metals, such as W, Mo, Ni, Ti, Cr, and V may also be used asthe outer layers in an emitter structure for the present invention.Insulators, and/or semiconductors, may also be used for the outer layersof the emitter multilayer structure, for example, to protect the emitterfrom oxidation. Useful materials for protective outer layers on theemitter include AIN, TiAIN, AlTiN, BN, TiN, SiN, SiC, diamond, andcarbon having a structure similar to diamond (ie., diamond-like carbon).In these embodiments, the outer layers can, but do not necessarily,protect the low work function emitter material against oxidation, sinceonly the emitting tip of the low work function emitter material needs tobe exposed.

[0053] As stated above, the actual emitting material itself maypreferably be any low work function material. Typical low work functionemitter materials include, but are not limited to, alkali metals such asLi, K Na, Rb, and Cs, alkaline earth metals such as Mg, Ba, Sr, and Ca,transition metals such as Y and Zr, and other metals such as Th, andalloys or compounds containing such materials, such as transition metalcarbides (e.g., ZrC, HfC, TiC or the like) and transition metal nitrides(e.g., ZrN, HfN, TiN or the like).

[0054] Typically, the emitter layer of the present invention has sharptips having a radius of curvature of about 20 nm or less, and more oftenof about 10 nm or less and most often about 5 nm or less.

[0055] As described in detail further below, the emitter of the presentinvention may, if desired, include nanofilaments having ends that serveas highly effective emission sites.

[0056] It is also within the scope of the present invention, althoughnot absolutely necessary, to include an optional layer between thesubstrate layer and the emitter and/or insulating layer. This optionallayer may be selected from various materials including insulatingmaterials, resistive materials, semiconducting materials, conductingmaterials, and combinations thereof. The advantages of including such anoptional layer include, e.g., promotion of better electrical contact oradhesion between emitter and substrate, incorporation of active orpassive current control elements for individual or group emitters,including transistors, p-n junctions, resistors, etc.

[0057] The gate layer may be a single layer, multilayer, composite,alloy, or elemental material. The gate should, however, include at leastone material that is a conductor, a semiconductor, or a resistivematerial. A resistive gate layer or a composite including a resistivematerial minimizes burnout by causing an IR drop that results in currentlimiting in the gate layer. Unlike the emitter and the substrate, thegate layer need not conduct electrons. That is, the gate may be aconductor by virtue of hole rather than electron mobility. The use of ap6 type semiconductor in the gate layer may be particularlyadvantageous, since it minimizes electrons from emitting from the gate,causing spurious and unregulated emission. Since the placement ofnanofilaments such as carbon nanotubes (either by adhesion or in-situgrowth) may not be with fine control, there may exist the probabilitythat some nanofilaments may physically touch and electrically short tothe gate layer. To possibly prevent this type of electrical shorting,the outer most layer of the multi-layer gate can be an insulator layer.With this, the standoff insulator is usually not recessed and usually isas tall as possible. Typically, any electrical interconnect to the gateis made through a via hole through the top insulator layer.

[0058] As mentioned previously, in one embodiment the perforation in thegate layer has an aperture that is larger than that of the underlyingfirst perforation in the insulating layer. In this way, the gate layeris blocked or shielded by the insulating layer(s) against any directline-of-sight with respect to the emitter. Thus, any emitted electronswould need to travel well above the substrate before there exists adirect line-of-sight from the electrons to the gate, at which point theelectrons would be under the influence of the potential created by theanode electrode. The perforation in the gate layer can have verticalsidewalls that are essentially parallel to the vertical sidewalls of theperforation in the insulating layer. Alternatively, the perforation inthe gate layer can define a tapered aperture that increases in size in adirection away from (i.e., normal to) the substrate layer. Otherarrangements and configurations would be readily apparent to one skilledin the art, given the disclosure of the present invention that the gatelayer be substantially blocked or shielded by the insulating layer froma direct line-of-sight with respect to the emitter.

[0059] As will be apparent, the starting template structure may have aconcentric, but not coincident, gate layer at an offset from the edge ofthe perforation in the insulating layer. This type of structure issuitable for a gate material that is difficult to directionally dryetch. Examples of such materials that are difficult to directionally dryetch include, but are not limited to, chrome and nickel. Alternatively,the gate material may be etchable, such materials including, but notlimited to, silicon and titanium nitride. If the gate material isetchable, a protection layer such as a chrome layer may be used toprotect the gate from being tapered. The protection layer with itsaperture coincident with the perforation in the insulating layer mayeither overhang or be coincident with the edge of the aperture of thegate layer. If desired, the protection layer may be removed by selectivewet etching after the etching back of the standoff layer.

[0060] Alternatively, a non-coincident starting template can beprovided. In such situations, the gate layer is usually etchableaccording to a dry or wet etching method. For these non-coincidentstarting templates, a structure comprising a substrate, insulator layer,a gate layer, and a protection layer is first provided. Then coincidentperforations are provided through the protection layer, gate layer,insulator layer and optionally part of the substrate. After that theperforation in the gate layer may be offset by using selective isotropicdry or wet etching.

[0061] In another embodiment, the perforation in the gate layer may becoincident with, or larger or small than, the perforation in theunderlying insulating layer. In this embodiment, however, the gate layeris substantially shielded from the emitter by using a standoff (orspacer) layer extending upward from at least within the perforation inthe insulating layer (i.e., the insulating layer that overlies theelectrically conductive substrate layer and underlies the gate layer).Typically, this standoff layer is essentially parallel to the verticalsidewalls of the perforation in the insulating layer and is disposedbetween the emitter and the vertical sidewalls of the perforation in theinsulating layer. In general, the upper portion of the shieldingstandoff layer will be the portion substantially shielding the gatelayer from the emitter.

[0062] The material for the standoff (or spacer) layer which blocks theline-of-sight between the emitter and the gate layer may be selectedfrom various materials including, but not limited to, silicon nitride,aluminum oxide, silicon dioxide, or any other good insulator that willnot be etched when etching the second vertical standoff (or spacer)layer described below. The material for the standoff (or spacer) layerwhich blocks the line-of-sight between the emitter and the gate layerpreferably (but not necessarily) is capable of holding a negativecharge.

[0063] In most cases, it is desirable to have a second vertical standoff(or spacer) layer extending between the first standoff or spacer layer(i.e., the shielding layer) and the emitter, usually extending tosomewhat less than the height of the emitter. Mainly, the secondstandoff layer provides mechanical support for the emitter and, togetherwith the first standoff layer, determines the distance between theemitter and the gate aperture edge. Any material may be used for thesecond standoff layer. For example, the second standoff layer may be aninsulator, a conductor, or a semiconductor. If the second standoff layeris an electron-conducting material, it can also serve as an electrontransport medium and heat sink to the emitter. If the second standofflayer is a resistive material, it can serve as a control mechanism forcurrent flow. The second standoff layer should be capable of beingetched without substantially etching the first standoff layer (i.e., thematerials for the first and second standoff layers are different).

[0064] The absolute and relative thicknesses of the various layers willdepend upon the intended use of the device. The best determination ofthese parameters for any known application may be determined by routineexperimentation combined with knowledge possessed by those havingordinary skill in the art of field emitter cells and arrays.Nevertheless, some additional guidance is offered here. A majoradvantage of the present invention is that the thicknesses of thevarious layers and component dimensions, such as emitter height, gateaperture size, gate-emitter separation, and insulator layer aperturesize are individually and independently selectable.

[0065] The base and the conductive part of the substrate of theinvention may be any thickness. In typical applications, the base andthe conductive part of the substrate will each be from about 0.5 μm toabout 1000 μm, and more often about 0.5 μm to about 100 μm. Typically,the insulating layer will have a thickness of about 0.1 m to about 10 μmand more often about 0.5 μm to about 10 μm. The gate layer typically hasa thickness of about 0.1 μm to about 1 μm. In the case where a standofflayer is used for shielding the gate layer from the emitter, theshielding standoff layer can have a thickness of about 100 Å to about0.1 μm. The second standoff layer, if used, may have a thickness ofabout 100 Å to about 1 μm. More often, the second standoff (or spacer)layer has a thickness of about 0.1 μm to about 0.5 μm.

[0066] It is often highly desirable to provide a conducting substratehaving a cavity with dimensions greater than that of the inner dimensionof the aperture defined by the standoff layer(s) at the bottom to extendthe depth and width of the emitter layer into the substrate in order toprotect the contacting portion of the emitter layer to the substrate andto provide a larger contact area between the emitter layer and thesubstrate. This cavity can be fabricated by selective etching of thesubstrate immediately after the standoff layer(s) has/have beendirectionally etched back.

[0067] If desired for handling or for a specific application, thesubstrate/insulator/emitter/gate (with or without standoff layer(s)) maybe supported upon a base. If used, the base may be any material,conductor, semiconductor, or insulator, or any combination of thesematerials.

[0068] Also, adhesion layers may be used, if needed, between theinsulating layer and the gate layer, between the emitter layer and thestandoff layer(s), between the insulating layer and the standofflayer(s), and/or between the emitter layer and the substrate, as well asbetween two layers of a multilayer component. Typical adhesion layersinclude Cr, Ti, and TiN. The adhesion layer may be included as a part(i.e., sublayer) of a multilayered substrate, insulating layer, spacerlayer, gate layer, and/or emitter layer.

[0069] In addition, the field emitter cell of the present invention caninclude, if desired, a protection layer on top of the gate layer duringfabrication of the field emitter cell. During fabrication, theprotection layer serves to prevent etching and tapering of the gatelayer. Where the gate layer is etchable by standard dry etching methods,the protection layer may be non-etchable by such methods so that aportion of the protection layer extends over the gate layer along theregion where the gate layer has been etched away. This provides addedprotection against the upper corner of the insulator aperture beingeroded during subsequent processing. In the case where the protectionlayer is initially coincident with the gate layer, the protection layercan remain and serve to strengthen the gate layer. If the protectionlayer is conductive and initially overhangs the gate layer, it should beremoved to achieve the off-set gate structure, prior to completion ofthe emitter cell fabrication.

[0070] Furthermore, although not absolutely necessary, the field emittercell of the present invention may optionally comprise remnants of anoverlayer, which is useful for the purpose of protecting the bottom andthe sidewall of the emitter from processing damage, such as from ionbeam sputtering or etching damage. If a metal overlayer such as tungstenis used, it can provide better electrical contact for the emitter aswell as current sharing and heat removal as compared to silicon orsilicon dioxide.

[0071] In all of the embodiments described herein, it may be desirableto include nanofilaments (nanowires or nanotubes) in the emitter filmlayer(s). These nanofilaments are usually hollow (nanotubes) or solid(nanowires), needle-like structures having a high aspect ratio.Typically, the nanofilaments have diameters from just a few up to abouttens of nanometers and lengths of up to about several microns. The mostwell known nanofilaments are carbon nanotubes, which can besingle-walled, multiple-walled, hollow or filled structures. Such carbonnanotubes are described in Dresselhaus, et al., Science of Fullerenesand Carbon Nanotubes, Academic Press, San Diego, Calif., 1996, which isincorporated herein by reference. Other nanofilaments usually compriseSi, SiC, TiC, NbC, Fe₃C, GaN, and the like, as well as combinationsthereof. If nanofilaments are used, it may be preferable to provide anoffset gate in order to minimize electrical shorts between the emitterfilm layer and the gate by the protruding nanofilaments. Carbonnanotubes and Si and SiC nanowires have been tested as ungated fieldemitter cells, and these cells have shown very promising field emissioncharacteristics attributed to the chemical stability of carbon,mechanical strength of the tube and the extremely small radius ofcurvature of the tube ends from which emission takes place. The sameadvantages would be expected when nanofilaments are incorporated intothe field emitter cells according to the present invention.

[0072] In all the embodiments where nanofilaments are used, the finalcell structures are more or less the same as those in whichnanofilaments are not used except that nanofilaments are attached to,embedded in, or sandwiched between the vertical emitter layers of thecells. The cells including the nanofilaments may be fabricated either tofeature a capping (or sandwiching) conducting protection layer for goodelectrical and mechanical contacts or without such a capping (orsandwiching) conduction protection layer. By using nanofilaments havinglengths which are a sizable fraction of the emitter film cylinderdiameter, the nanofilaments will end up with a large vertical component(i.e., parallel to the emitter film layer) due to the stronger overalladhesion or contact area of the filament in this orientation. Thus,there will be many nanofilaments with ends protruding over the top edgeof the emitter film layer, and these ends will serve as highly effectivefield emission sites. The vertical positions of these nanofilaments arelikely to be uneven which, as mentioned previously, calls for gates thatare offset to avoid electrical shorting.

[0073]FIG. 1 shows a side view of a typical field emitter cell 10according to the present invention. Substrate 12 has a depression 14with essentially vertical sidewalls. Insulator layer 16 directlyoverlays substrate 12. Multilayer gate 18 directly overlays insulatorlayer 16, and has a perforation 30 that defines a tapered aperture.Insulator layer 16 has therein a perforation 20, with vertical sides,extends upward from depression 14. Substrate 12 therefore defines thebottom of perforation 20. Multilayer emitter 22 extends, essentiallyvertically upward from the bottom of perforation 20 to the vicinity ofan edge of insulator layer 16. A spacer 24 extends vertically betweeninsulator layer 16 and multilayer emitter 22, but well short of the topof multilayer emitter 22. Although not required, depression 14 providesphysical support and better electrical contact for multilayer emitter22. A vacuum gap 26 exists between the upper portion of the emitter andinsulator layer 16. If desired, insulating layer 16 may be undercut aslong as sufficient top portion remains so that there is no direct lineof sight between the emitter layer and the gate layer. FIG. 2 shows atop view of the device shown in FIG. 1.

[0074]FIG. 1A shows an alternative, preferred embodiment which is thesame as the embodiment shown in FIGS. 1 and 2, except substrate 12 has adepression 14 a with an enlarged cavity 80 (i.e., the cavity has lateraldimensions greater than those of the aperture defined by the standofflayer(s)). Thus, as explained previously, the substrate 12 can have adepression that has an enlarged cavity with dimensions greater thanthose of the inner dimension of the aperture defined by the standofflayer(s) at the bottom to extend the depth and the width of the emitterinto the substrate in order to provide a larger contact area between theemitter layer and the substrate. It is to be understood that theenlarged cavity can have a shape other than those shown in the figures,as long as the objects of the present invention are met.

[0075] Further, it is to be understood that the substrate 12 can beprovided with the above-described enlarged cavity for all embodiments ofthe present invention, including the embodiments shown in FIGS. 8-24 and28-31.

[0076] A field emitter cell according to the present invention, or arraythereof, may be produced by a variety of methods. In one typicalprocedure, shown in FIGS. 3 through 7, conducting substrate 12, with orwithout a base (not shown), is provided on at least its upper surface(with respect to any base that may be present) with insulating layer 16and multilayer gate 18 overlaying insulating layer 16. The insulatinglayer may be provided by any means, such as oxidation of the substratelayer, bonding of a preformed insulating layer, CVD deposition, CBDdeposition, physical deposition such a evaporation or sputtering,ion-implantation, etc. Likewise, the method of providing the gate layeris not particularly critical to the present invention. Methods such asmelt bonding of a preformed layer of conducting material, evaporativedeposition, CVD (chemical vapor deposition), CBD (chemical beamdeposition), electroplating, electroless plating, sputter deposition,and ion-implantation may be used. If a gate layer etchable by standarddirectional dry etching is used, the gate layer is desirably etchable bythe same directional dry etch method (reactive ion etching (RIE) or ionbeam sputtering) to be used for removing the standoff insulator layer.

[0077] Perforation 20 forms a well that extends at least to the uppersurface of the conducting substrate. A variety of methods may be used toprovide the needed perforations in the gate layer and the insulatinglayer. These methods plating/lift-off. The reactive ion etching methodinvolves the reactive ion etching of the laminate (e.g., Sisubstrate/SiO₂ insulating layer/p-type Si layer) through alithographically patterned mask. The ion beam milling method may becarried out on a structure comprising, e.g., a resist mask on Cr, whichin turn is on a p-type Si layer. The lift-off method involves providinga solid post of resist, which can be removed following gate materialdeposition on the surface around (and on top of) the post. Theplating/lift-off method involves the use of electroless plating of a Nilayer on a catalyst comprising Pd particles selectively bonded to asilane (e.g., aminosilane) monolayer of the self-assembled type aroundresist posts, which method is described in detail in U.S. Pat. Nos.5,079,600 and 5,077,085, both of which are incorporated herein byreference. In another known method, perforations can be made bymechanical stamping, using, for example, the method described by StephenChou, Science, Vol. 272, 5 April 1996, pages 85 through 87, the entiretyof which is incorporated herein by reference. In an alternative method,posts, for example of Si, may be provided on the substrate, for exampleby RIE. Then, an insulator layer is deposited over the post structureand substrate such that insulator thickness is greater than the heightof the post. The resulting structure is then planarized, mechanicallypolished, or chemically-mechanically polished (CMP) to provide a flatupper surface. Selectively etching back of the insulator leaves aportion of the post protruding above the insulator layer. Then,directional deposition of a gate material over the top of the post andthe substrate is performed. The resulting pillar or post may then bepreferentially etched to provide a hole, with essentially verticalsidewalls, through the gate layer and insulator layer. As shown in FIG.3, the aperture of the gate film is initially coincident with thetemplate hole, which includes the perforation 20 within insulator layer16.

[0078] If desired, a standoff layer 24 is then deposited by any suitablemethod, e.g., chemical vapor deposition, under conditions that produce athinner layer thickness at the corner 32 of the template hole than atareas on top away from the hole, as shown in FIG. 4. The horizontalportions of the standoff layer can be removed by directional etching. Aslight overetching ensures that the bottom of the template hole iscleared of the standoff layer and has a depression as shown in FIG. 5.At this point the portion of the gate layer near the edge of theoriginal template hole is largely removed or is significantly thinnerthan before.

[0079] As shown in FIG. 5A, it may be preferable to use selectiveetching to produce a larger depression or cavity at the bottom of thetemplate hole to permit the extension of the depth and the width or onlythe width of the emitter into the substrate in order to provide a largercontact area between the emitter layer and the substrate. It is to beunderstood, of course, that the enlarged depression or cavity can beused, and is preferred, in all embodiments of the present invention,including the embodiments shown in FIGS. 8-24 and 28-31.

[0080] Next, as shown in FIGS. 6 and 6A, an emitter film (or multilayersandwich films, not shown, but as described in U.S. application Ser. No.09/045,853, which is incorporated herein by reference) is depositedpreferably by a conformal method, e.g., chemical beam deposition orchemical vapor deposition, followed by the optional deposition of aconsiderably thicker overlayer 34 of a preferentially removable material(e.g., an insulator, semiconductor or a conductor). Examples of suchpreferentially removable overlayer materials include silicon dioxide,silicon, tungsten, molybdenum, as well as any material readily removableby RIE or wet etching processes. As mentioned above, the purpose of theoverlayer is to protect the bottom and sidewall of the emitter from ionbeam sputtering or etching damage. If the template hole is small enough(i.e., less than 1 micron), it is advantageous to fill the perforationwith the overlayer material. The horizontal and corner portions of theoverlayer and the emitter layer are then removed by directional etching.The upper portions of the resulting standoff layer and the overlayer canbe selectively removed (e.g., hydrofluoric acid in the case of silicondioxide) so as to let the vertical emitter hydrofluoric acid in the caseof silicon dioxide) so as to let the vertical emitter edge protrudeabove the remaining standoff layer and the overlayer, as shown in FIGS.7 and 7A. However, the recessing of the standoff layer should be done soas to not significantly alter the substrate insulator material (inparticular at the corner 28. For example, the insulator can be siliconnitride or thermal silicon dioxide, while the standoff layer can be CVDsilicon dioxide. Hydrofluoric acid would attack the CVD silicon dioxideat a much faster rate than it would the thermal silicon dioxide or thesilicon nitride. Thus, the edge of the gate layer is tapered due to thevarying thickness of the deposited standoff layer near the corner of thetemplate hole.

[0081]FIGS. 8 and 9 show an alternative embodiment in which the startingtemplate structure has a concentric, but not coincident, multilayer gate18 at an offset from the edge of perforation 20 in the insulating layer16. Thus, the perforation 30 of the gate layer is larger than theperforation 20 of the insulating layer. The processing steps, which areillustrated in FIGS. 10 through 14, are similar to those shown in FIGS.3 through 7. In particular, FIG. 10 shows the offset position of thegate layer, which is not etchable by standard RIE methods; FIG. 11 showsthe deposition of the standoff layer; FIG. 12 shows the etching back ofthe standoff layer, FIG. 13 shows the deposition of the emitter layerand overlayer, and FIG. 14 shows the directional etch removal andrecessing of the remaining standoff layer and overlayer.

[0082] Specifically with respect to FIGS. 10-14, the starting templatestructure has a concentric, but not coincident, multilayer gate 18 at anoffset from the edge of the perforation 20 in the insulating layer 16,as shown in FIG. 10. As discussed above, this type of structure issuitable for a gate material that is difficult to directionally dryetch. Examples of such materials that are difficult to directionally dryetch include, but are not limited to, chrome and nickel. Again, it ispreferable to enlarge the depression at the bottom of the template holein the substrate after the etch back of the standoff layer usingselective etching (without affecting other components) to enhanceelectrical contact between the substrate and the emitter.

[0083] Alternatively, as shown in FIGS. 15-19, the gate material may beetchable by standard RIE methods, such materials including, but notlimited to, silicon. If the gate material is etchable, a protectionlayer 36, such as a chrome layer, may be used to protect the gate frombeing tapered, as shown in FIGS. 15 and 16. The protection layer 36 mayeither overhang to be coincident with the perforation 20 in theinsulating layer 16 or be coincident with the edge of the perforation 30in the gate layer. If desired, the protection layer may be removed bywet etching or selective wet etching after the etching back of thestandoff layer. As in FIGS. 10-14, the processing steps shown in FIGS.15-19 are similar to those described in the previously mentionedembodiments, leading to a final emitter cell which has a non-tapered,offset gate layer that can be made of a material etchable by standardRIE methods.

[0084] In yet another embodiment, FIGS. 20-24 show a gate layer 18having a perforation 30 that is coincident with perforation 20 in theinsulator layer 16. It is to be understood, however, that it is alsowithin the scope of the present invention to alternatively provide aperforation in the gate layer that is either smaller or larger than theperforation in the insulator layer. Two standoff layers, 38 and 39,which can be made of different materials are deposited by any suitablemethod, e.g., CVD. In FIG. 22, both standoff layers are directionallyetched back so that their vertical top ends are about the same height asthe gate layer 18, with standoff layer 39 covering the edge of theperforation 30 in the gate layer. At this point, an enlargement of thedepression at the bottom of the hole may be carried out as in thepreviously described embodiments to provide improved electrical contactbetween the emitter and the substrate, if desired. Then, as shown inFIG. 23, the multilayer emitter 22 and overlayer 34 are deposited bysimilar methods as in the previously described embodiments. Afterdirectional removal of the horizontal portions of the multilayer emitter22 and overlayer 34, only the standoff layer 38 and the overlayer 34 arerecessed back (without etching or altering standoff layer 39) to providethe final emitter cell as shown in FIG. 24. For example, standoff layer39 can be made of silicon nitride, while standoff layer 38 can be madeof CVD silicon dioxide. Standoff layer 39 therefore shields the edge ofmultilayer gate 18 from any direct line of sight from the emitter 22.Covering the exposed sharp edges of multilayer gate 18 can alsopotentially reduce the possibility of arcing.

[0085] FIGS. 28-31 show the embodiments where nanofilaments 50 areincluded in the field emitter cell, with or without a conductingprotection layer 55 as part of the emitter layer structure.Specifically, FIGS. 28 and 29 show the cross-sectional and top views ofa cell without a capping (or sandwiching) conducting protection layerfor the nanofilaments; FIGS. 30 and 31 show the cross-sectional and topviews of a cell in which a capping (or sandwiching) conductingprotection layer 55 is included. As mentioned previously, the conductionprotection layer (e.g., Pt), which is optional, ensures good electricaland mechanical contacts between the nanofilaments and the emitter layer.

[0086] The fabrication processing steps for cells includingnanofilaments are essentially the same as those for the otherembodiments of the present invention up to the deposition of the emitterlayer at which point the incorporation of the nanofilaments take place.A layer, preferably made of a noble metal (e.g., Pt, Au, or Au/Pd), isdeposited as or on an existing emitter layer 22 (or the exposed layer ofa multilayer emitter structure), preferably conformally, so that thevertical sidewalls of the emitter layer are coated. The structure isthen sputtered by an ion beam or reactive-ion-etched normal to thesubstrate in order to remove the emitter layer(s) from the tophorizontal surface of the emitter cell. The structure is then treatedwith a solution of, e.g. a long chain thiol such as octanedecanethiolwhich will selectively attach to the noble metal film left on thevertical sidewalls of the emitter layer, rendering it coated with ahydrophobic monolayer of alkanethiol. As described in M. Burghard etal., “Controlled Adsorption of Carbon Nanotubes on Chemically ModifiedElectrode Arrays,” Advanced Materials, 10, pp. 584-587 (1998), which isincorporated herein by reference, nanofilaments (specifically carbonnanotubes) can be treated with a micellar surfactant such as sodiumdodecylsulfate (SDS), which attach to the nanofilament in a shell, toprovide negatively-charged end groups that project outward to the waterphase, which end groups will preferentially attach to a hydrophobic(e.g., octadecanethiol-coated noble metal) surface. It should be notedthat in a neutral or basic pH aqueous solution, silicon dioxide surfacesare negatively charged. According to Burghard et al., supra, SDS-coatedcarbon nanotubes are repelled by and would not attach to such negativelycharged surfaces.

[0087] Therefore, by applying a suspension of the SDS-coatednanofilament in such a solution to the structure, the negatively chargedSDS-coated nanofilaments selectively adhere to the hydrophobicalkanethiol-coated noble metal sidewalls of the emitter and is repelledby negatively charged surfaces (e.g., silicon layer gate with nativeoxide, CVD silicon dioxide standoff layer, and the exposed silicondioxide insulator). By initially choosing nanofilaments having lengthsof a sizable fraction of the diameter of the emitter film cylinder(e.g., 10-50%), the nanofilaments are likely to preferentially orientthemselves parallel to the longitudinal walls of the emitter layer orhave a significant vertical component in this orientation. The shorternanofilaments (e.g., length/diameter ratios less than 0.2) are lesslikely to cause shorting of the gate. A careful rinse will remove thenanofilaments from the horizontal top surface of the emitter cellwithout removing them from the emitter layer sidewall. Optionally, toensure that the carbon nanotubes are removed from the horizontal topsurface, a directional oxygen RIE step can be carried out afteretch-back of a protection layer such as CVD silicon dioxide which may bedeposited to protect the nanotubes on the vertical sidewall of theemitter layer. Optionally, the CVD silicon dioxide protection layer maybe stripped by dipping in HF. As described above, an optional conductingprotection film such as an oxidation resistant metal (e.g., Pt) can bedeposited preferably conformally on top of the resulting structure inorder to provide better electrical contact and stronger mechanicalanchoring. In addition, an optional low work function material such asLi can be deposited before the deposition of the protection layer.Further, an overlayer as in the other embodiments of the presentinvention can optionally be deposited before a second sputtering step.Finally, the structure can then again be ion beam sputtered normal tothe substrate to remove the conducting protection film from thehorizontal surface as well as to exposed fresh ends of upward orientednanofilaments. Then the exposed vertical standoff layer, if present, canbe recessed as in the other embodiments of the present invention.

[0088] Alternatively, instead of adhering the nanofilaments to thesidewalls of the emitter film, nanofilaments can be grown in situ on thesidewalls. In such an embodiment, however, the sidewalls of the emittermust be a catalytic metal such as Ni, Fe, and/or Co, which metals can bedeposited by using suitable precursors such as metal carbonyls employinga CVD method, or by physical evaporation or sputtering.

[0089] Alternatively, the catalyst metal can be the outer layer of amulti-layer emitter structure, e.g., the sidewall and substrate could befirst coated with an inner conductive layer, followed by an intermediatelayer (buffer layer), and then the catalyst layer. The same proceduresare used to remove the emitter layers from the top horizontal surface,leaving them intact on the sidewall and optionally intact on thesubstrate on the bottom of the aperture. The inner layer can enhanceelectrical conduction between the substrate and the nanofilaments. Thebuffer layer can serve multiple functions including providing strongadhesion between two layers, acting as a diffusion barrier to preventintermixing by diffusion between two layers, and protecting thesubstrate or inner layer from chemical reactions. Such a buffer layeracting as diffusion barrier may be needed between the inner conductivefilm and the catalyst film to prevent alloying between the films whichwould lose the catalytic action for growing the nanofilaments. Bufferlayer examples include: carbon, silicon carbide, transition metalcarbides, transition metal nitrides, transition metal silicides,conducting oxides(such as TiO, RuO₂), and certain transition metals(such as Ti, Ta, and Hf). Sometimes the buffer layer by itself (withoutthe inner conductive layer), can be effectively used as the interfacelayer between the catalyst layer and the substrate/insulator (i.e.,preventing the catalyst layer diffusing into the substrate/insulator orprotecting the substrate from chemical attack such as oxidation). Thisis followed by removal of the multilayer emitter structure layer fromthe top horizontal surface as before. A number of methods such aspyrolysis, plasma CVD, and arc discharge can be used to selectively grownanofilaments, especially carbon nanotubes, on these catalytic metalsurfaces. Any residual carbon impurities on the horizontal surfaces canbe removed by using directional oxygen RIE. The standoff layer may notbe recessed if the multilayer emitter structure layer is too thin.

[0090] Having described the invention, the following examples are givento illustrate specific applications of the invention including the bestmode now known to perform the invention. These specific examples are notintended to limit the scope of the invention described in thisapplication.

EXAMPLES Example 1 Sample GSO4 (with Tapered Gate Layer)

[0091] A silicon (100) wafer was thermally oxidized to produce a 400 nmthick silicon dioxide layer (insulator layer) on top of the wafer. A 150nm thick, heavily doped p-type amorphous silicon layer (gate layer) wasdeposited on top of the silicon dioxide. The wafer was then patterned bystandard lithographic means and reactive ion etched (RIE) to producedholes with a diameter of about 630 nm and a depth of about 800 nm. Theholes have essentially vertical sidewalls. The holes were configured insmall linear arrays of 50 in 1×1 cm fields on the wafer. The wafer wascut into these 1×1 cm square pieces for processing.

[0092] On sample wafer GSO4, a 290 nm thick layer (measured on topsurface of the sample) of silicon dioxide (standoff layer) was depositedby low pressure CVD. It was etched back completely by RIE and the RIEwas continued to remove at least 20 nm of the substrate silicon at thebottom of the hole.

[0093] Adhesion and emitter layers were then deposited by chemical beamdeposition (a very low pressure limit of CVD). The sample was heated to430° C. in a high vacuum chamber and placed at 3 mm distance in front ofa doser tube for Cr deposition with chromium carbonyl as the Crprecursor. A few hundred Angstroms of Cr was deposited. In the same run,a sandwich layer of Pt/Li/Pt was deposited at 297° C. by dosing with4×10⁻⁶ Torr of tetrakis-trifluorophosphine platinum, followed by t-butyllithium, and again tetrakis-trifluorophosphine platinum, all in thepresence of 1.0×10⁻⁵ Torr of hydrogen, for durations of 22 min, 8 min,and 20 min, respectively.

[0094] An overlayer of silicon dioxide about 100 nm thick was thendeposited on top of the emitter layer by low pressure CVD.

[0095] The sample was then sputtered with a 3-cm diameter argon ion beamat a beam energy of 400 eV, beam current of 10 mA, and beam angle normalto the substrate for 19 minutes to remove the silicon dioxide overlayer,the multi layer Pt/Li/Pt/Cr from the top horizontal surface. Then thesample was dipped in a 2.5% buffered HF solution for 18 seconds torecess the vertical portions of the silicon dioxide standoff layer andthe overlayer.

[0096] SEM analysis of GSO4 revealed a field emitter cell (FIG. 25)having a hollow vertical cylindrical emitter (cathode) with cylinderwall thickness of about nm and an outer diameter of about 250 nm,centered in a thermal silicon dioxide aperture with diameter of 630 nm,concentrically surrounded by a tapered gate (p-type amorphous silicon)with inner and outer diameters (starting and finish of the taperedregion) of 870 nm and 1100 nm, respectively. The top edge of the emitterwas below the top level of the thermal silicon dioxide aperture, sothere was no direct line of sight to the gate.

[0097] The sample was mounted in a emission test apparatus in aultrahigh vacuum chamber with the sample 2 mm away from an anode plate.With the emitter (substrate) grounded through a 10 M-ohm series ballastresistor and the anode plate biased at +400V, a positive bias voltagewas applied to the gate to extract emission. The resulting anode andgate currents (emission currents collected on the anode and the gate)versus gate voltage characteristics are shown in FIG. 26. The salientfeature is the extremely low gate current (<0.2% of the anode current).

Example 2 Sample G3R (p-type Silicon Gate with Top Cr Protection Layer)

[0098] The starting structure was similar to sample GSO4 in the firstexample, except that there was a 600 A thick Cr protection layer on topof the p-type silicon gate layer and that the hole diameter in thethermal silicon dioxide was about 1.2 microns. The edge of the p-typesilicon gate near the hole opening was offset by about 150 nm by usingisotropic dry etching. A standoff silicon dioxide layer 480 nm thick onthe top surface was deposited using low pressure CVD. It was etched backusing RIE and continued so that at least several hundred Angstroms ofthe substrate silicon is removed from the bottom of the hole. A furtherenlargement of the bottom silicon substrate area was carried out withselective isotropic etching to produce a larger contact surface areabetween the emitter and the substrate. Using similar chemical depositionprocedures as in example #1, a sandwich emitter layer consisting ofPt/Li/Pt was deposited. For this sample, no overlayer was used. Ion beamsputtering using similar parameters removed the emitter layer from thetop horizontal surface. Buffered HF dip was again used to recess the topportion of the exposed vertical standoff layer. Electron emissionmeasurements were obtained in a similar manner as in Example 1. Theapplied anode voltage was +300V. The current-voltage characteristics areshown in FIG. 27 which shows a gate current less than 1% of the anodecurrent.

What is claimed is:
 1. A method of making a field emitter cell,comprising: providing an insulating layer on an upper surface of anelectrically conductive substrate layer; forming an electricallyconductive gate layer on an upper surface of said insulating layer;forming at least one perforation through said gate layer that extendsdownwardly into said insulating layer, said at least one perforationhaving essentially vertical sidewalls; forming a standoff layer on saidgate layer and said vertical sidewalls under conditions that produce athinner layer thickness near the upper corners of said perforation andthen directionally removing the horizontal portions of said standofflayer and a portion of said gate layer near the upper corners of saidperforation to produce a tapered gate layer; overetching the standofflayer slightly to ensure complete removal of said standoff layer fromthe bottom of said perforation and the formation of a depression in saidsubstrate layer; depositing a multilayer emitter over said standofflayer and said gate layer; removing at least the horizontal portions ofsaid multilayer emitter and recessing the top vertical portion of saidstandoff layer so as to permit the edges of the retained multilayeremitter to protrude above the remaining standoff layer and the remainingoverlayer; and removing a further portion of said gate layer so thatthere is no direct line of sight between said multilayer emitter and anypart of said gate layer.
 2. A method for making a field emitter cellcomprising: providing a template structure comprising an electricallyconductive substrate layer, an insulating layer with a first perforationhaving vertical sidewalls directly on said substrate layer, and anelectrically conductive gate layer with a second perforation over saidinsulating layer, said second perforation being offset and larger thansaid first perforation in aperture size; forming a standoff layer onsaid gate layer and said vertical sidewalls and then removing thehorizontal portions of said standoff layer; overetching the standofflayer from the bottom of said first perforation and the formation of adepression in said substrate layer; depositing a multilayer emitter oversaid standoff layer and said gate layer; and removing at least thehorizontal portions of said multilayer emitter and recessing the topvertical portions of said standoff layer so as to permit the edges ofthe retained multilayer emitter to protrude above the remaining standofflayer.
 3. A method for making a field emitter cell comprising: providinga template structure comprising an electrically conductive substratelayer, an insulating layer with a first perforation having verticalsidewalls directly on said substrate layer, and an electricallyconductive gate layer with a second perforation over said insulatinglayer, said second perforation being coincident with, or larger orsmaller than, said first perforation; forming a first standoff layerover said gate layer; forming a second standoff layer over said firststandoff layer; removing the horizontal portions of said second standofflayer and said first standoff layer so that the vertical top endsthereof are approximately the same height as said gate layer;overetching said standoff layers slightly to ensure complete removal ofsaid standoff layers from the bottom of said first perforation and theformation of a depression in said substrate layer; depositing amultilayer emitter over said second standoff layer and said gate layer;removing the horizontal portions of said multilayer emitter; andselectively removing the upper portions of said second standoff layer,without removing any part of said first standoff layer.
 4. A method ofmaking a field emitter cell, comprising: providing an insulating layeron an upper surface of an electrically conductive substrate layer;forming an electrically conductive gate layer on an upper surface ofsaid insulating layer; forming at least one perforation through saidgate layer that extends downwardly into said insulating layer, said atleast one perforation having essentially vertical sidewalls; forming astandoff layer on said gate layer and said vertical sidewalls underconditions that produce a thinner layer thickness near the upper cornersof said perforation and then directionally removing the horizontalportions of said standoff layer and a portion of said gate layer nearthe upper corners of said perforation to produce a tapered gate layer;overetching the standoff layer slightly to ensure complete removal ofsaid standoff layer from the bottom of said perforation and theformation of a depression in said substrate layer; depositing amultilayer emitter over said standoff layer and said gate layer;removing at least the horizontal portions of said multilayer emitter;and removing a further portion of said gate layer so that there is nodirect line of sight between said multilayer emitter and any part ofsaid gate layer.
 5. A method for making a field emitter cell comprising:providing a template structure comprising an electrically conductivesubstrate layer, an insulating layer with a first perforation havingvertical sidewalls directly on said substrate layer, and an electricallyconductive gate layer with a second perforation over said insulatinglayer, said second perforation being offset and larger than said firstperforation in aperture size; forming a standoff layer on said gatelayer and said vertical sidewalls and then removing the horizontalportions of said standoff layer; overetching the standoff layer from thebottom of said first perforation and the formation of a depression insaid substrate layer; depositing a multilayer emitter over said standofflayer and said gate layer; and removing at least the horizontal portionsof said multilayer emitter.
 6. The method of claim 32, furthercomprising the steps of: forming a protection layer over said gate layerprior to said overetching step; and removing said protection layer.
 7. Amethod for making a field emitter cell comprising: providing a templatestructure comprising an electrically conductive substrate layer, aninsulating layer with a first perforation having vertical sidewallsdirectly on said substrate layer, and an electrically conductive gatelayer with a second perforation over said insulating layer, said secondperforation being coincident with, or larger or smaller than, said firstperforation; forming a first standoff layer over said gate layer;forming a second standoff layer over said first standoff layer; removingthe horizontal portions of said second standoff layer and said firststandoff layer so that the vertical top ends thereof are approximatelythe same height as said gate layer; overetching said standoff layersslightly to ensure complete removal of said standoff layers from thebottom of said first perforation and the formation of a depression insaid substrate layer; depositing a multilayer emitter over said secondstandoff layer and said gate layer; and removing the horizontal portionsof said multilayer emitter.
 8. The method of claim 1, wherein saidmultilayer emitter includes a catalytic metal layer and furthercomprising the step of growing a nanofilament on said catalytic metallayer.
 9. The method of claim 8, wherein said nanofilament is a carbonnanotube.
 10. The method of claim 1, further comprising the step ofselectively removing a further portion of said substrate layer near saiddepression.
 11. The method of claim 1, further comprising the steps of:depositing an overlayer over said multilayer emitter; and removing thehorizontal portion of said overlayer.
 12. The method of claim 1, furthercomprising the step of adhering a nanofilament on said multilayeremitter.
 13. The method of claim 12, wherein the nanofilament is acarbon nanotube.
 14. The method of claim 8, wherein said multilayeremitter further includes a buffer layer.
 15. The method of claim 14,wherein said buffer layer is selected from the group consisting ofcarbon, silicon carbide, transition metal carbides, transition metalnitrides, transition metal silicides, conducting oxides, Ti, Ta, and Hf.16. The method of claim 1, wherein said gate layer comprises amultilayer.
 17. The method of claim 16, wherein said multilayer includesan insulator.
 18. A field emitter cell comprising: an electricallyconductive substrate layer; an insulating layer directly upon saidelectrically conductive substrate layer, said insulating layer having afirst perforation therethrough, said first perforation having anaperture, at least one essentially vertical sidewall and a bottomsurface defined by said electrically conductive substrate layer; anelectrically conductive multilayer gate directly upon said insulatinglayer, said electrically conductive multilayer gate having a secondperforation therein, said second perforation having an aperture largerthan the aperture of said underlying first perforation; and anelectrically conductive multilayer emitter, electrically insulated fromsaid multilayer gate and in electrical contact with said substratelayer, said multilayer emitter extending upward from said substrate,said multilayer emitter having an upper electron-emitting edge in closeproximity to said multilayer gate layer.
 19. The field emitter cell ofclaim 18, wherein said multilayer emitter includes a catalytic metal anda buffer layer.
 20. The field emitter cell of claim 18, wherein ananofilament is in electrical contact with said multilayer emitter. 21.The field emitter cell of claim 18, wherein said multilayer gateincludes an insulator layer.
 22. The method of claim 35, furthercomprising the steps of: forming a protection layer over said gate layerprior to said overetching step; and removing said protection layer. 23.The method of claim 2, wherein said multilayer emitter includes acatalytic metal layer and further comprising the step of growing ananofilament on said catalytic metal layer.
 24. The method of claim 3,wherein said multilayer emitter includes a catalytic metal layer andfurther comprising the step of growing a nanofilament on said catalyticmetal layer.
 25. The method of claim 4, wherein said multilayer emitterincludes a catalytic metal layer and further comprising the step ofgrowing a nanofilament on said catalytic metal layer.
 26. The method ofclaim 5, wherein said multilayer emitter includes a catalytic metallayer and further comprising the step of growing a nanofilament on saidcatalytic metal layer.
 27. The method of claim 7, wherein saidmultilayer emitter includes a catalytic metal layer and furthercomprising the step of growing a nanofilament on said catalytic metallayer.
 28. The method of claim 2, further comprising the step ofselectively removing a further portion of said substrate layer near saiddepression.
 29. The method of claim 2, further comprising the steps of:depositing an overlayer over said multilayer emitter; and removing thehorizontal portion of said overlayer.
 30. The method of claim 2, furthercomprising the step of adhering a nanofilament on said multilayeremitter.
 31. The method of claim 3, further comprising the step ofselectively removing a further portion of said substrate layer near saiddepression.
 32. The method of claim 3, further comprising the steps of:depositing an overlayer over said multilayer emitter; and removing thehorizontal portion of said overlayer.
 33. The method of claim 3, furthercomprising the step of adhering a nanofilament on said multilayeremitter.
 34. The method of claim 4, further comprising the step ofselectively removing a further portion of said substrate layer near saiddepression.
 35. The method of claim 4, further comprising the steps of:depositing an overlayer over said multilayer emitter; and removing thehorizontal portion of said overlayer.
 36. The method of claim 4, furthercomprising the step of adhering a nanofilament on said multilayeremitter.
 37. The method of claim 5, further comprising the step ofselectively removing a further portion of said substrate layer near saiddepression.
 38. The method of claim 5, further comprising the steps of:depositing an overlayer over said multilayer emitter; and removing thehorizontal portion of said overlayer.
 39. The method of claim 5, furthercomprising the step of adhering a nanofilament on said multilayeremitter.
 40. The method of claim 7, further comprising the step ofselectively removing a further portion of said substrate layer near saiddepression.
 41. The method of claim 7, further comprising the steps of:depositing an overlayer over said multilayer emitter; and removing thehorizontal portion of said overlayer.
 42. The method of claim 7, furthercomprising the step of adhering a nanofilament on said multilayeremitter.
 43. The method of claim 2, wherein said gate layer comprises amultilayer.
 44. The method of claim 3, wherein said gate layer comprisesa multilayer.
 45. The method of claim 4, wherein said gate layercomprises a multilayer.
 46. The method of claim 5, wherein said gatelayer comprises a multilayer.
 47. The method of claim 7, wherein saidgate layer comprises a multilayer.